1. Field of the Invention
The present invention relates to a magnetic sensor for the purpose of reading an information signal recorded onto a magnetic medium.
2. Related Art
In the prior art, there has been disclosure of a magnetic reading transducer called a magnetoresistive (MR) sensor or head, and it is known that this is capable of reading data from a magnetic surface with a high linear density.
An MR sensor detects a magnetic field signal by converting it to a change in resistance as a function of the strength and direction of magnetic flux sensed by a reading element. This MR sensor according to the prior art operates based on the anisotropic magnetoresistive (AMR) effect, whereby one component of the resistance of the reading element is proportional to cosine of the angle between the direction of magnetization and the direction of the detected current flowing in the element.
A detailed description of the AMR effect is found in D. A. Thompson et al “Memory Storage and Related Applications” IEEE Trans. on Mag. MAG-11, P. 1039 (1975).
In a magnetic head using the AMR effect, to suppress Barkhausen noise vertical bias is often applied, and there are cases in which FeMn, NiMn, or a nickel oxide or the like is used as a bias application material.
Additionally, there is a more prominent magnetoresistive effect, in which the change in resistance of a laminated magnetic sensor is attributed to spin-dependent transmission of conduction electrons between magnetic layers via a non-magnetic layer, and to spin-dependent scattering at the accompanying layer boundary.
This magnetoresistive effect is known variously as the “giant magnetoresistive” effect or “spin valve effect.” Such a magnetoresistive sensor is made of an appropriate material, and exhibits a better sensitivity and larger resistance change than seen with a sensor using the AMR effect.
In this type of MR sensor, the internal planar resistance between a pair of ferromagnetic layers separated by a non-magnetic layer changes in proportion to the cosine of the angle between the magnetization directions of the two layers.
In the Japanese Unexamined patent publication (KOKAI) No. 2-61572, bearing a priority date of June 1988, there is language describing a laminated structure that yields a large MR effect occurring by virtue of anti-parallel arrangement of magnetization within a magnetic layer. Materials that can be used in this laminated structure are listed in the above-noted specification as ferromagnetic transition metals and alloys.
There is also disclosure of a structure in which there is the addition of a layer that fixes at least one of the ferromagnetic layers separated by an intermediate layer, and that FeMn is suitable for used as that layer.
In the Japanese Unexamined patent publication (KOKAI) No. 4-358310, bearing a priority date of Dec. 11, 1990, there is disclosure of an MR sensor having two thin-film ferromagnetic layers partitioned by a thin-film non-magnetic metal layer, wherein when the applied magnetic field is zero the magnetization directions of the two ferromagnetic thin-film layers are perpendicular, the resistance between two non-coupled ferromagnetic layers changing in proportion to the cosine of the angle between the magnetization directions of the two layers, this being independent of the direction of current flow in the sensor.
In the Japanese Unexamined patent publication (KOKAI) No. 4-103014, related to a patent application filed Aug. 22, 1990, there is language describing a ferromagnetic tunnel effect film, wherein in a multilayer ferromagnetic tunnel junction element in which an intermediate layer is interposed between ferromagnetic layers, and wherein a bias magnetic field is applied to at least one ferromagnetic layer from an anti-ferromagnetic material.
In the Japanese Unexamined patent publication (KOKAI) No. 10-162327, bearing a priority date of Nov. 27, 1996, there is language describing a magnetoresistive effect head structure using a ferromagnetic tunnel junction in the magnetic sensor section.
In the past, although an NiFe alloy has been used in the lower shield of a playback head using a ferromagnetic tunnel junction, even with an NiFe film of approximately 1 μm fabricated using sputtering, the crystal grains thereof are 20 nm or larger, resulting is great roughness, the average surface roughness value Ra measured using an AFM (atomic force microscope) being 3 nm or greater.
In the case of fabricating the NiFe using plating, the Ra value is double or more of the case of using sputtering. In the case of a ferromagnetic tunnel junction head (hereinafter referred to as a TMR head), the roughness in the barrier layer part of the TMR film is greatly influenced by the roughness of the layer immediately therebelow.
In a TMR head, the TMR film is either formed directly on the lower shield or formed on the lower shield with an intervening gap insulation layer or lower conductor layer formed on the lower shield, and if the roughness of the lower shield is excessive, the surface of the gap insulation layer or the lower conductor layer formed thereover will inherit this roughness.
In either case, this leads to an increase in the barrier layer roughness.
Even in the case in which the roughness of the lower shield is small, however, if the roughness of the gap insulation layer or lower conductor layer formed thereover is great, this also results in an increase in the roughness of the barrier layer in the TMR film. While in the past, aluminum was used for the lower conductor layer, in the case of a low-melting-point metal such as aluminum, because the crystal grain diameter is 20 nm or greater, the surface roughness has an Ra value of 3 nm or greater.
The problem that arises when the barrier layer roughness is large is that, in the TMR material the effective film thickness within the barrier layer becomes smaller.
As shown in FIG. 2 of the accompanying drawings, the tunnel transition probability of an electron in the TMR film passing through the barrier layer is greatly dependent upon minute changes in the barrier layer film thickness, and when the barrier layer effective film thickness exhibits a distribution, there is a tendency for electrons to concentrate in a location that has even a slightly smaller effective film thickness.
Given the above, not only does the junction resistance decrease, but also the current is concentrated in a location in which the effective film thickness is small, resulting in localized increases in voltage applied to the barrier layer, thereby increasing the dependency of the resistance change on the sensing current, and reducing the amount of resistance change compared to the case in which the sensing current is constant.
Additionally, there is a reduction in resistance change attributed to the thinning of the effective thickness of the barrier layer. With a decrease in the effective film thickness also comes an increase in the ferromagnetic coupling passing from the fixed layer to the free layer via the barrier layer.
Because the ferromagnetic coupling is an important factor in establishing the operating point of a magnetoresistive effect element, if this is excessively large, it is not possible to establish an appropriate operating point, resulting in the problems of a worsening of waveform asymmetry and reduction in output upon playback.
In addition to the above, in the Japanese examined patent publication (KOKOKU) No. 8-21166, the Japanese Unexamined patent publication (KOKAI) No. 5-217123, and Japanese patent number 2651015, there are disclosures of the configuration of magnetic heads. However, while the structure of these heads is described, there is in no case a disclosure of a technical configuration that smoothes the surface of the film component directly below the barrier layer in a compound-type magnetoresistive effect element.
In the Japanese Unexamined patent publication (KOKAI) No. 9-270321, while there is language describing the use of a amorphous component for the purpose of smoothing the surface of a soft magnetic shield layer, thereby improving the insulation of the insulation film, there is no language with regard to solving the problem of ferromagnetic coupling in the barrier layer of a compound-type magnetoresistive effect element.
In the Japanese Unexamined patent publication (KOKAI) No. 11-316919, although there is indication of the use of an amorphous alloy in the lower shield layer in a magnetic head using a compound-type magnetoresistive effect element, this is no more than a suggestion of the possibility of such use, and there is no disclosure of a technology to make the surface roughness of the barrier layer small.
Accordingly, it is an object of the present invention to improve on the above-noted drawbacks of the prior art, by providing a magnetoresistive effect sensor and a method for manufacturing a magnetoresistive effect sensor, wherein, by reducing the surface roughness in the barrier layer of a TMR film, playback waveform symmetry is maintained, while avoiding a reduction in output.